Air has long been separated into its component parts by cryogenic rectification. In such process, the air is compressed, purified and cooled within a main heat exchanger to a temperature suitable for its rectification and then introduced into an air separation unit having higher and lower pressure columns that operate at higher and lower pressures, respectively to produce nitrogen and oxygen-rich products. Additionally, the air separation unit can also include an argon column to separate argon from an argon-rich stream withdrawn from the lower pressure column.
The air, after having been cooled, is introduced into the higher pressure column to produce an ascending vapor phase that becomes evermore rich in nitrogen to produce a nitrogen-rich vapor overhead that is condensed to produce nitrogen-rich liquid streams that reflux both the higher and the lower pressure columns and thereby initiate the formation of the descending liquid phase within each of such columns. The descending liquid phase becomes evermore rich in oxygen as it descends to produce bottoms liquids in each of the columns that are rich in oxygen. An oxygen-rich liquid that collects within the lower pressure column as the bottoms liquid is reboiled to initiate formation of an ascending vapor phase within such column. Such reboiling can be brought about by condensing the nitrogen-rich vapor overhead of the higher pressure column to produce the nitrogen-rich reflux streams.
A stream of the oxygen-rich bottoms liquid of the higher pressure column, known in the art as crude liquid oxygen or kettle liquid, is utilized to introduce an oxygen-rich liquid stream into the lower pressure column for further refinement. Streams of nitrogen-rich vapor and residual oxygen-rich liquid that is not vaporized in the lower pressure column can be introduced into the main heat exchanger to help cool the incoming air and then be taken as products. An argon-rich stream can be removed from the lower pressure column and further refined in an argon column or column system to produce an argon-rich stream. In all such columns, mass transfer contacting elements such as structured packings, random packings or trays can be used to bring the liquid and vapor phases into intimate contact to conduct the distillation occurring within such columns.
It is known that as the liquid phase descends in the higher pressure column, that it will not only become evermore rich in oxygen, but also krypton and xenon. Due to the low relative volatility of krypton and xenon, only the bottom several stages will have appreciable concentrations of krypton and xenon. In order to concentrate the krypton and xenon, it is also known to provide a mass transfer contacting zone below the point at which the crude liquid oxygen stream is taken to wash krypton and xenon from the incoming air. For example, in DE 100 00 017 A1, an air separation plant is disclosed in which the air after having been fully cooled is introduced into the bottom of a higher pressure column having such a mass transfer contacting zone built into the bottom of the higher pressure column to produce a bottoms liquid that is rich in krypton and xenon. A stream of such bottoms liquid is then introduced into a rectification column to produce an oxygen-rich vapor overhead that is reintroduced into the higher pressure column and a crude krypton-xenon bottoms liquid that can be taken and further refined. Similarly, in US 2006/0021380, a stream of bottoms liquid rich in krypton and xenon is produced in a mass transfer contacting zone built into the bottom of the higher pressure column. The bottoms liquid is then introduced into a distillation column positioned on the top of the argon column. A condenser for the argon column reboils such distillation column to produce a residual liquid further enriched in krypton and xenon. A stream of the residual liquid is then stripped within a stripping column to produce a krypton-xenon enriched bottoms liquid that can be further refined.
As will be discussed, the present invention, among other advantages, provides an air separation method in which more krypton is able to be efficiently recovered from the incoming air than in the prior art patents discussed above.